In an Alternate Polymer™ co-extrusion system, two or more extruders deliver two or more plastic melts to gear pumps connected at the output of the extruders. The outputs of the gear pumps are connected to a co-extrusion die. The gear pumps are controlled to alternately deliver more or less of their corresponding melt, requiring the gear pumps to vary their operating speed. Pressure sensors between each gear pump and its associated extruder detect any abrupt pressure differential across the gear pump. Such a pressure differential indicates that the gear pump's speed has changed, and that the extruder should change its speed to ensure proper feed of the plastic melt from the extruder to the gear pump. Alternate Polymer systems of this kind are described in the U.S. Pat. Nos. 5,725,814 and 5,695,789, issued Mar. 10, 1990 and Dec. 9, 1997, respectively in the name of Holton E. Harris and assigned to the assignee of the present invention. These two patents are incorporated herein by reference.
If an extruder can respond to a speed change in its associated gear pump, the plastic melt will flow without significant problems. However, an extruder in operation has certain inertia which resists a speed change in the extruder. Thus, large extruders have a significant amount of inertia, which prevents them from quickly adjusting to speed changes in the gear pump. Generally, in practice, an extruder can adjust to the speed changes in the gear pump where is the extruder has a relatively small diameter such as up to 1 inch or 1.5 inch diameter. However, in extrusions systems where the extruder has a larger diameter, such as, for example, in extrusion systems that produce corrugated tubing, the extruder may not be able to adjust effectively to speed changes in the gear pump. This results in the problem of having too much or too little plastic melt supplied to a gear pump from the extruder, which prevents the effective operation of the extrusion system.